1. Field of the Invention
This invention relates to a thin-film transistor that uses a semiconductor film of amorphous silicon (a-Si). More particularly, it relates to a technology for the prevention of a decrease in the offresistance of a thin-film transistor due to light from a back light positioned at the back of the thin-film transistor in the case where the thin-film transistor is used as a dispaly device provided with liquid crystal panels.
2. Description of the Prior rt
In recent years, there has been a good potential market for the active-matrix display devices, as large-scale display devices that use liquid crystals, etc., in which thin-film transistors made with the use of a semiconductor film of a-Si are formed in a matrix on an insulating substrate such as glass, etc.
FIG. 16 shows a conventional thin-film transistor Tr using a semicondctor film of a-Si wherein an active layer 4a disposed on a gate insulating film 3a is much broader than a gate electrode 2a positioned below the gate insulating film 3a, and moreover either the active layer 4a or an n+-a-Si semiconductor film forming both source and drain regions 6a and 7a is made without consideration of its thickness. A protective insulating film 5a is disposed on the active layer 4a.
In the case where the thin-film transistor Tr constitutes a display device with liquid crystals, a back light is placed at the insulating substrate (glass plate 1a) side. When the thin-film transistor Tr is off (i.e., negative voltage is applied to the gate electrode 2a), carriers, (such as electrons and their related holes) are generated due to light from the back light, in the portion of the active layer 4a that is not in alignment with the gate electrode 2a, resulting in a decrease in the resistance of the thin-film transistor Tr at the time when the thin-film transistor Tr is off. Thus, the thin-film transistor Tr does not function as a switching device.
In order to solve this problem, the thickness of the active layer 4a of an a-Si semiconductor film can be thinned. For example, when the thickness thereof is set to be 100 .ANG. or less, the influence of the back light on the active layer 4a is not observed. However, if the active layer 4a is made too thin, then the resistance of the thin-film transistor Tr becomes unacceptably high when the transistor Tr is turned on.
The generation of carriers in the active layer 4a can be also prevented by the formation of an optical shield in the thin-film transistor Tr, which causes an increase in the number of production steps, making yields low and increasing the production cost.
Moreover, the thin-film transistor Tr can be designed such that the active layer 4a is positioned at a portion of the gate insulating layer 3a corresponding to the gate electrode 2a and is formed into the same shape and size as the gate electrode 2a by a common mask-alignment technique. However, according to such a technique, alignment errors arise unavoidably and side-etchings must be carried out, which makes the size of the thin-film transistor large, resulting in a decrease in the ratio of the surface area of the picture-element electrode to the surface area of the liquid-crystal display panel and an increase in the load capacity between the gate electrode and the drain electrode. Accordingly, the enlargement of the surface area of the display device cannot be attained.